Existing wind turbines include an inverter that regulates ac voltage in synchronism with the AC grid, connected via a step-up transformer operating at the grid frequency. FIG. 1 shows a typical such grid connected wind power generating system 10. The system includes a wind generator 12, a generator side converter 14 and a grid side converter 16. The system further includes a grid side controller 18, a generator side controller 20 and a power grid 22. A direct current (DC) link 28 connects the generator side converter 14 and the grid side converter 16. The generator side converter 14 converts alternating current (AC) power generated by the wind generator 12 into DC power. The grid side converter 18 then converts the DC power to AC power at a frequency compatible with the power grid 22.
The combination of the grid side controller 18 and grid side converter 16 functions as a current source for the grid 22. In other words, the grid side controller 18 controls the phase and amplitude of the output current of grid side converter 16. The generator side controller 20 generates switching signals for the generator side converter 14. In some embodiments, a turbine controller 38 provides a torque reference T to the generator side controller 20 based on wind velocity or rotor speed of the wind turbine. The turbine controller 38 generates the torque reference such that maximum energy is captured from the wind using, in part, Maximum Power Point Tracking (MPPT) circuitry. Active power control of the wind power generating system 10 of FIG. 1 maintains the output current of the wind turbine 42 by adjusting the phase angle of the AC voltage applied to the grid 22 via transformer 50.
Recently it has become advantageous in certain instances to convert the output previously applied to the AC distribution grid 22 to DC for transmission from, for example, offshore wind farms via subsea cables. In exemplary configurations, the need to convert the normally generated AC voltage to DC for transmission is met by coupling rectifiers to the output side of transformer 50 at the generator site and coupling the rectifier output to a DC transmission line. The addition of further components at the generator site, particularly in the case of offshore installations, however, presents additional challenges at least in view of the additional space and cost required for such devices.
Thus, there is a need for a method and system to reduce the number of conversion components and costs required to produce direct current at a suitable level for application to a DC transmission line.